Riesch, Rüdiger, Barrett-Lennard, Lance G., Ellis, Graeme M., Ford, John K.B.
and Deecke, Volker B. (2012) Cultural traditions and the evolution of
reproductive isolation: ecological speciation in killer whales? Biological Journal
of the Linnean Society, 106 . pp. 1-17.
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Running head: Population divergence in killer whales
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Cultural traditions and the evolution of reproductive isolation:
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ecological speciation in killer whales?
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Rüdiger Riesch1*, Lance G. Barrett-Lennard2, Graeme M. Ellis3, John K. B. Ford3,
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Volker B. Deecke4,5
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1
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University, 127 David Clark Labs, Raleigh, NC 27695-7617, USA
2
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Department of Biology & W. M. Keck Center for Behavioral Biology, North Carolina State
Cetacean Research Laboratory, Vancouver Aquarium, PO. Box 3232, Vancouver, BC, V6B
3X8, Canada
3
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Cetacean Research Program, Pacific Biological Station, Fisheries and Oceans Canada,
Nanaimo, British Columbia V9T 6N7, Canada
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4
Centre for Wildlife Conservation, University of Cumbria, Newton Rigg, Penrith, CA11 0AH, UK
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5
Sea Mammal Research Unit, Scottish Oceans Institute, University of St. Andrews, St. Andrews,
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Fife, KY16 8LB, Scotland UK
*
corresponding author’s address: North Carolina State University, Department of Biology, 127
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David Clark Labs, Raleigh, NC 27606-7617, USA; email:
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[email protected]
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1
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Human evolution has clearly been shaped by gene–culture interactions, and there is growing
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evidence that similar processes act on populations of non-human animals as well. Recent
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theoretical studies have shown that culture can be an important evolutionary mechanism due to
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the ability of cultural traits to spread rapidly both vertically and horizontally, resulting in
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decreased within-group variance and increased between-group variance. Here, we collate the
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extensive literature on population divergence in killer whales (Orcinus orca) and argue that they
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are undergoing ecological speciation as a result of dietary specializations. While we cannot
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exclude the possibility that cultural divergence predates ecological divergence, we propose that
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cultural differences in the form of learned behaviors between ecologically-divergent killer whale
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populations have resulted in varying degrees of reproductive isolation both in sympatry and
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allopatry and thus have the potential to lead to incipient speciation even despite ongoing low-
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levels of gene flow.
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ADDITONAL KEYWORDS: Cetacea – culture – cultural evolution – gene-culture coevolution
– vocal dialects
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2
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THE ROLE OF CULTURE IN EVOLUTION
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Recent scientific advances have revolutionized our understanding of how gene-culture
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coevolution has shaped and influenced human evolution (e.g., Richerson & Boyd, 2005; Laland,
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Odling-Smee & Miles, 2010). Traditionally, it was thought that products of culture (e.g. shelter,
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clothing, and tools) helped humans shield themselves from other selective pressures, thus
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slowing down the rate of human adaptive evolution considerably. However, most scientists now
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agree that culture can also have the opposite effect of increasing the rate at which many human
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traits evolve as well as influencing the direction of selection acting on human populations
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(Richerson & Boyd, 2005; Hawks et al., 2007; Laland et al., 2010). For example, the cultural
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practice of cattle farming in some human cultures drove the evolution of lactose tolerance in
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adults (Simoons, 1978; Richerson & Boyd, 2005; Laland et al., 2010) and humans from
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agricultural populations living on high-starch diets have a higher number of amylase gene copies
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(e.g. Perry et al., 2007).
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Consequently, there has been a recent push for interdisciplinary projects that integrate
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archeological and anthropological data with discoveries of human genetics and evolutionary
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theory to further our understanding of human evolution (Laland et al., 2010). We propose that
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similar processes are also acting on populations of non-human animals and have far greater
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evolutionary consequences than previously assumed. In this review we will use the example of
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the killer whale (Orcinus orca) to illustrate how cultures and behavioral traditions can drive
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population divergence even in the absence of geographic isolation.
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3
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ECOLOGICAL SPECIATION AND KILLER WHALES
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As an alternative to the allopatric model of speciation as a result of geographic isolation (e.g.
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Coyne & Orr, 2004), the idea that reproductive isolation and ultimately speciation can result
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from ecologically-based divergent selection has recently received considerable attention (e.g.
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Rundle & Nosil, 2005; Schluter, 2009). This model is referred to as ecological speciation and can
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apply to allopatric, parapatric, and sympatric populations, as long as divergent selection is the
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ultimate cause of both population differentiation and reproductive isolation (Rundle & Nosil,
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2005). Strong evidence for ecological speciation has come from a variety of natural systems,
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laboratory experiments, comparative studies, and theoretical models (reviewed in Rundle &
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Nosil, 2005; Schluter, 2009).
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Here, we evaluate the evidence that the largest member of the highly diverse family
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Delphinidae, the killer whale, is currently undergoing ecological speciation. Although generally
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regarded as a single species (Committee on Taxonomy, 2009; but see LeDuc, Robertson &
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Pitman, 2008; Morin et al., 2010), numerous ecologically divergent killer whale lineages are
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being recognized throughout its global distribution (three in the North Pacific, four around
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Antarctica, and at least two in the North Atlantic; see below for more details).
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Rundle and Nosil (2005) listed three components necessary for demonstrating ecological
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speciation: (1) an ecological source of divergent selection between populations, (2) a mechanism
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responsible for reproductive isolation, and (3) a genetic mechanism linking divergent selection to
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reproductive isolation. Here we begin by summarizing the evidence for population divergence
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among killer whales and outline the sources of divergent selection that appear to be responsible
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for population divergence. We then discuss potential causes of reproductive isolation and suggest
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that cultural traditions and cultural inheritance play a central role. Finally, we argue that the
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deterministic mechanism linking divergent selection to reproduction in killer whales may be
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cultural rather than genetic.
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EVIDENCE FOR POPULATION DIVERGENCE AMONG KILLER WHALES
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Killer whales are second only to humans as the most widely distributed mammal on earth. They
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inhabit all the world’s oceans but greatest densities can be found in temperate and polar regions
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(Ford, 2009). Killer whales are the top predator in many marine ecosystems. As a species, their
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diet includes over 140 species of fish, squid, mammals, reptiles and birds, but different killer
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whale populations typically show a high degree of dietary specialisation (Ford, 2009).
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In areas of high marine productivity, two or more ecologically specialized lineages, or
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ecotypes, of killer whales often co-occur in sympatry. Ecotypes show differences in diet,,
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movement patterns, pigmentation, behavior, and morphology, as well as in group size, stability
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and composition (Boran & Heimlich, 1999; Baird, 2000; Barrett-Lennard & Heise, 2006; Ford,
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2009; Ford & Ellis, in press; see Table 1).
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Sympatric ecotype assemblages are currently reported from three different geographical
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regions: the eastern North Pacific, the eastern North Atlantic, and Antarctica, but there is also
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recent evidence for sympatric ecological divergence of killer whales from around New Zealand,
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the Russian Far East, the western North Atlantic, and the eastern tropical Pacific (Reeves et al.,
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2004; Fig. 1). Lack of evidence for additional killer whale ecotypes from other parts of the
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world’s oceans is most likely an artifact of the geographically-biased research efforts.
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Probably best studied are the three sympatric ecotypes inhabiting the waters of the
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eastern North Pacific: the so–called offshores, residents, and transients (Boran & Heimlich,
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1999; Baird, 2000; Barrett-Lennard & Heise, 2006; Ford & Ellis, in press). Several
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geographically distinct populations exist for each ecotype. The resident ecotype, for example
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consists of the Southern resident population (mainly southern British Columbia and Washington
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State, but also Oregon and California), the Northern resident population (mainly northern British
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Columbia but also southeastern Alaska and Washington State; Fig. 2), and the South Alaskan
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resident population (Barrett-Lennard & Heise, 2006). Recent evidence suggests that there are
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additional resident-like populations near the Aleutian Islands (Matkin et al., 2007), in the Bering
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Sea (Hoelzel et al., 2007), and in the western North Pacific around Kamchatka (Ivkovich et al.,
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2010). The transient ecotype is composed of the West Coast transient population (Fig. 2), the
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Gulf of Alaska transient population, and the ‘AT1 transient’ population of Prince William
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Sound, Alaska (Barrett-Lennard & Heise, 2006), with further putative transient populations in
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the Bering Sea and around the Aleutian Islands. Residents specialize on fish, in particular salmon
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(Oncorhynchus spp.), while transients prey on marine mammals and occasionally seabirds
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(Barrett-Lennard & Heise, 2006; Ford, 2009; Ford & Ellis, in press). Offshores probably form a
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single population ranging from the Aleutian Islands to California. Their full dietary spectrum and
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their preferred prey have yet to be fully described; however, preliminary observations suggest
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that they also specialize on fish, e.g. sharks and Pacific halibut (Hippoglossus stenolepis; Jones,
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2006; Krahn et al., 2007; Dahlheim et al., 2008; Ford et al., 2011; Ford & Ellis, in press).
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Besides dietary specializations, these three killer whale ecotypes also differ in a variety of other
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traits including group size and the degree of philopatry (Table 1).
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Research suggests that up to five sympatric killer whale ecotypes are found in Antarctic
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waters: Type A, Type B (with a large and small form), Type C, and Type D (Fig. 3; Pitman &
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Ensor, 2003; Pitman, 2011; Pitman et al., 2011). Again, there appears to be a general pattern of
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specialization on either marine mammals and birds or fish, but the dietary distinctions warrant
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further investigation (Pitman & Ensor, 2003; Andrews, Pitman & Balance, 2008). Type A killer
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whales appear to almost exclusively prey on Antarctic minke whales (Balaenoptera
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bonaerensis), while the large form of Type B (i.e., the ‘pack ice killer whale’) specialize on seals
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and the small form (i.e., the ‘Gerlache killer whale’) forages for penguins (Pitman, 2011; Fig. 3).
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Type C killer whales, on the other hand, specialize on Antarctic toothfish (Dissostichus
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mawsoni) and show what is probably the most pronounced morphological divergence within
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killer whales, with both sexes being considerably smaller than other killer whales (Pitman et al.,
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2007; Pitman, 2011; Table 1; Fig. 3). Finally, a recent study by Pitman et al. (2011) described
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the sub-Antarctic killer whale, Type D killer, which clearly differs from the other Antarctic killer
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whales in pigmentation and morphology (Fig. 3). However, the available data are insufficient to
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identify whether Type D also has a specific dietary niche.
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Finally, around Iceland, Shetland, and Norway, killer whales have been reported to
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diverge in trophic ecology as well, but again the actual dietary specializations are not yet clearly
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resolved (Fig. 2). While some killer whales in the North Atlantic specialize on marine mammals
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(Foote et al., 2009; Deecke et al., 2011), others specialize on fish (predominantly herring,
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Clupea harengus; Similä & Ugarte, 1993; Similä, Holst & Christensen, 1996). Stable isotope
144"
analysis of individuals with the same mitochondrial haplotype suggests that some feed at
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different trophic levels (Foote et al., 2009), which could either point to a greater niche width for
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these populations or could be an indication that this dietary shift is relatively recent so that not
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enough time has passed for it to have resulted in a genetic signature as well. More research is
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warranted to resolve this important issue,
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7
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BEHAVIORAL DIVERGENCE AND KILLER WHALE CULTURES
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Culture has been defined in various ways over the past decades, but the most common definitions
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characterize culture as population-level behavioral variation that can be transmitted vertically
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(from parent to offspring), oblique (from any member of an older generation to any member of a
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younger generation), and horizontally (within the same generation) via social learning (Laland &
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Janik, 2006; Danchin & Wagner, 2008; Laland & Galef, 2009). After decades of scientific
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debate, there is still disagreement regarding the existence of cultures in non-human animals (e.g.,
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Laland & Galef, 2009), but some of the most famous examples proposed as animal cultures
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include tool-use in chimpanzees, Pan troglodytes, orangutans, Pongo spp., and bottlenose
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dolphins (Tursiops spp.), as well as songs in songbirds and humpback whales, Megaptera
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novaeangliae (Whiten et al., 1999; Krützen et al., 2005; Laland & Janik, 2006; Lalang & Galef,
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2009; Garland et al., 2011). Behavioral diversity likely to represent cultural variation has also
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been described in killer whales, and comprises traits such as acoustic communication, social
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behavior, and foraging strategies (Table 2).
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The best example for such behavioral variation in killer whales comes from residents and
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transients off Alaska, British Columbia, and Washington State, which have been studied
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intensively since the 1970s (e.g. Bigg, 1982). This research has documented a complete lack of
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dietary overlap between the two ecotypes (based on observational studies, stomach content
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analysis of stranded individuals, and stable isotope analysis of blubber biopsies): in over 40 years
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of study, no resident has ever been found to kill and consume marine mammal prey, and no
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transient has been documented to take fish (e.g., Ford et al. 1998; Saulitis et al., 2000; Herman et
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al., 2005; Dahlheim & White, 2010). These dietary specializations result in differences in social
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structure between residents and transients with transient group sizes usually being much smaller
8
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than resident group sizes (Bigg et al., 1990; Baird & Dill, 1996; Ford & Ellis, 1999; Baird,
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2000).
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AN EXAMPLE OF CULTURE: VOCAL BEHAVIOR IN KILLER WHALES
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Killer whales produce three types of sounds: echolocation clicks are thought to function in
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orientation and prey detection whereas pulsed calls and whistles are communicative signals
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(Ford, 1989; Thomsen, Franck & Ford, 2002). While there is recent evidence for some universal
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acoustic signals (Rehn et al., 2011), the structure and the frequency of use of most vocalizations
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differs strikingly between ecotypes. In the northeastern Pacific for example, mammal-hunting
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transient killer whales use echolocation very sparingly whereas residents, sympatric salmon
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specialists, echolocate frequently (Barrett-Lennard, Ford & Heise, 1996). The same applies for
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pulsed calls (Deecke, Ford & Slater, 2005) and whistles (Riesch & Deecke, 2011) – transients
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produce these almost exclusively in non-hunting contexts. Whereas salmon cannot detect killer
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whale sounds over significant distances, marine mammals have good underwater hearing and
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exhibit anti-predator behavior in response to transient calls (Deecke, Slater & Ford, 2002).
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Mammal-eating killer whales therefore probably reduce their vocal behavior to avoid
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eavesdropping by potential prey, and similar behavioral patterns have recently been revealed in
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mammal-hunting killer whales in the North Atlantic (Deecke et al., 2011).
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In addition to usage differences, the structure of killer whale vocalizations also varies
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among populations and social groups. Differences in the frequency content and patterning of
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echolocation clicks are probably related to the nature of the echolocation task and to prey hearing
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(Barrett-Lennard et al., 1996), and thus directly related to feeding. Many pulsed calls and some
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whistles are highly stereotyped and can easily be assigned to call or whistle types (Ford, 1989;
9
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Riesch, Ford & Thomsen, 2006). Variation in communicative sounds can be found on three
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levels. First, there is geographic variation among allopatric populations, because spatially
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separated populations differ in their repertoires of stereotyped pulsed calls, e.g., eastern North
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Pacific (Ford, 1989, 1991; Yurk et al., 2002; Deecke et al., 2005), western North Pacific
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(Filatova et al., 2004), Norway (Strager, 1995), and Antarctica (Richlen & Thomas, 2008), and
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whistles (Riesch et al., 2006; Samarra et al., 2010). Second, dialect variation exists among
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sympatric and parapatric populations because populations of different ecotypes with overlapping
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home ranges also do not usually share call or whistle types. The same is true for populations of
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the same ecotype with adjacent or partially overlapping home ranges (Moore et al., 1988; Ford,
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1991; Riesch et al., 2006; Riesch, Ford & Thomsen, 2008; Riesch & Deecke, 2011). Third, there
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is often dialect variation among social groups within a population. The Northern resident
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population for example contains three acoustic clans (Ford, 1991). Members belonging to
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different clans do not share any call types but associate frequently. They do however share
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stereotyped whistles (Riesch et al., 2006, 2008). Call types shared within a clan typically show
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matriline-specific variation in call structure (Miller & Bain, 2000; Nousek et al., 2006; Deecke et
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al., 2010).
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CULTURAL TRANSMISSION
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The central requirement of culture is that it must be transmitted through social learning (Laland
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& Janik, 2006; Danchin & Wagner, 2008, 2010; Laland & Galef, 2009). In killer whales,
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behavioral and genetic studies have provided evidence that vocal dialects are transmitted via
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vocal learning rather than genetic inheritance (reviewed in Rendell & Whitehead, 2001; Janik,
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2009). There is further evidence that vocal learning is not limited to vertical transmission from
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mother to offspring, but also takes place horizontally between matrilines with similar and
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different dialects (Deecke, Ford & Spong, 2000; Janik, 2009; Weiß et al., 2010). Specialized
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hunting techniques of certain killer whale populations, such as the intentional stranding observed
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at Península Valdés, Argentina, and in the Crozet Islands (Table 2), have also been suggested to
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be transmitted via social learning from generation to generation (Hoelzel, 1991; Guinet &
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Bouvier, 1995), although experimental data on this are not yet available.
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GENETIC DIVERGENCE, ECOTYPE FORMATION AND REPRODUCTIVE
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ISOLATION
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Similar to various other large mammals, such as cheetahs, Acinonyx jubatus (O’Brien et al.,
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1983), Antarctic fur seals, Arctocephalus gazella (Hoffman et al., 2011), or humans (Li &
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Durbin, in press), killer whales are characterized by low worldwide genetic diversity both in
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mitochondrial and microsatellite DNA (Hoelzel et al., 2002; Morin et al., 2010). While such a
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reduction in genetic diversity is often the result of small effective population sizes and historical
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bottlenecks (Hoelzel et al., 2002), ‘cultural hitchhiking’ has been proposed as an additional
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hypothesis to explain low mtDNA diversity in matrilineal odontocetes such as killer whales
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(Whitehead, 1998; but see Mesnick et al., 1999).
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Overall, there is no evidence for a correlation between categories of preferred dietary
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items and genotypes on a global scale, which suggests that local prey specializations had
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multiple independent origins (Hoelzel et al., 2002; Morin et al., 2010; Foote et al., 2011b). In
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fact, both Morin et al. (2010) and Foote et al. (2011b) suggested that, based on mtDNA analyses,
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killer whales might have repeatedly migrated between the North Pacific and the North Atlantic;
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forming first the North Pacific transients, which are the sister group to all other killer whales.
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Residents and offshores, however, originate from subsequent colonization events (Morin et al.,
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2010; Foote et al., 2011b); in other words, ecological divergence of offshores, residents, and
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transients could have occurred during either the allopatric or the sympatric phase, while
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Antarctic killer whale ecotypes are more likely to have ecologically diverged in sympatry
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(LeDuc et al., 2008; Morin et al., 2010; Foote et al., 2011b).
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Both mtDNA and nuclear DNA analyses confirm profound genetic differentiation of
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killer whale ecotypes albeit to different degrees (e.g., Hoelzel, Dahlheim & Stern, 1998; Barrett-
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Lennard, 2000; Hoelzel et al., 2007; Morin et al., 2010; Pilot, Dahlheim & Hoelzel, 2010; Foote
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et al., 2011c). Mitochondrial DNA suggests relatively old divergence times between ecotypes
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with the oldest (i.e., transients) ca. 700,000 years or 40,000 generations ago (Morin et al., 2010;
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Foote et al., 2011b; Table 1). These estimates are comparable to divergence times between
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several evolutionarily young cetacean species (e.g., the divergence between the three right whale
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species (Eubalaena spp.) within Mysticeti, the divergence between the Ganges River and the
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Indus River dolphin (Platanista spp.) within Odontoceti, or even the divergence between the
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long-finned and short-finned pilot whale (Globicephala spp.) within Delphinidae; McGowen,
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Spaulding & Gatesy, 2009). On the other hand, divergence between different populations of the
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same ecotype appears to be more recent (e.g., Hoelzel et al., 2007).
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Population genetic patterns as revealed by microsatellite analyses are not as
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straightforward. In the eastern North Pacific, genetic distance based on nuclear DNA is greatest
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between resident and transient ecotypes, and FST-values clearly suggest some degree of
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reproductive isolation between the two (e.g., Hoelzel et al., 1998, 2007; Morin et al., 2010; Pilot
263"
et al., 2010). A similar pattern of reproductive isolation has been described between the
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Antarctic Type A killer whales and both Type B and C ecotypes (LeDuc et al., 2008). On the
12
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other hand, genetic distances between other ecotypes are smaller, and interbreeding appears to
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occur more often, for example between eastern North Pacific transients and offshores, eastern
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North Pacific residents and offshores, (Barrett-Lennard, 2000; Hoelzel et al., 2007; Morin et al.,
268"
2010; Pilot et al., 2010), or Antarctic Type B and Type C killer whales (LeDuc et al., 2008).
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How can these slightly contradictory population genetic patterns be explained? In over 35
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years of study, no case of between-culture dispersal or immigration has ever been recorded
271"
(J.K.B. Ford and G. M. Ellis, unpublished data), and even genetic analyses do not
272"
(unequivocally) suggest permanent between-ecotype dispersal (e.g., Hoelzel et al., 2007; Pilot et
273"
al., 2010). However, gene flow is likely to occur during rare temporary associations between
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individuals of different ecotypes (see discussions in Hoelzel et al., 2007; Pilot et al., 2010). Since
275"
offspring from such matings would then be born into and raised by their mothers’ social group,
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we would not expect these matings to leave a mtDNA signature, but they would be detectable
277"
using nuclear markers.
278"
As a consequence of long-term divergence and several fixed haplotype differences in
279"
mtDNA, recent studies have suggested different species or subspecies designations for certain
280"
killer whale ecotypes (LeDuc et al., 2008; Morin et al., 2010); however, based on the reported
281"
patterns of gene flow uncovered by nuclear markers we are not convinced that there is currently
282"
enough unambiguous evidence to warrant the designation of different killer whales species or
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subspecies.
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13
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CULTURAL DIVERGENCE, TYPES OF REPRODUCTIVE ISOLATION, AND
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SPECIATION
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Recent studies have shown that pre-mating reproductive isolation can arise between different
288"
cultures even in the absence of genetic change, ultimately driving speciation (theoretical model:
289"
Gavrilets, 2004; conceptual studies: Danchin & Wagner, 2008, 2010). Some of the best empirical
290"
evidence for this comes from Darwin’s finches (Geospiza spp.), in which song, a culturally
291"
inherited trait, is an important factor in species recognition and mate choice (e.g., Grant & Grant,
292"
1996, 2009, 2010). Males usually learn the song from their fathers, but females prefer song types
293"
different from those of their fathers, creating an effective barrier against inbreeding (Grant &
294"
Grant, 1996). This means that a culturally inherited trait could play a crucial role in speciation by
295"
promoting genetic isolation on secondary contact (Grant & Grant 1996, 2009, 2010). We suggest
296"
that, similar to the situation in Darwin’s finches, divergent cultural evolution has led to pre-
297"
zygotic and, at the very least, non-genetic post-zygotic reproductive isolation among certain
298"
killer whale populations or ecotypes, although reproductive isolation is clearly not complete (see
299"
also Boran & Heimlich, 1999; Baird, 2000).
300"
There are at least four hypothetical mechanisms (both pre- and post-mating reproductive
301"
barriers) by which cultural divergence can lead to reproductive isolation in killer whales. First,
302"
culture could act through xenophobia if cultural imprinting is so strong that mating will not take
303"
place between individuals belonging to different cultural groupings (Danchin & Wagner, 2010).
304"
For killer whales, we propose a mechanism of positive and negative sexual/cultural imprinting
305"
acting on two levels in opposite directions that is similar to that described for humans (Rantala &
306"
Macrinkowska, 2011): On the basal level, acoustic similarity via, for example, matrilineal vocal
307"
dialects within certain populations can act as an acoustic family badge that reveals genetic
14
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relatedness (e.g., Deecke et al., 2010). Negative assortative mating by dialect has been shown in
309"
some populations (Barrett-Lennard, 2000) and means that animals can reliably avoid mating with
310"
close maternal kin and thus control and reduce inbreeding processes. On the higher level, other
311"
cultural characteristics (e.g., group behavior, social structure, dietary specializations, or vocal
312"
differences between populations; reviewed in Barrett-Lennard & Heise, 2006; Ford & Ellis, in
313"
press) could act as a cultural badge that promotes endogamy within the cultural grouping by
314"
determining who an individual socializes and mates with. This process could be reinforced by
315"
differences in habitat use that limit the potential for direct interaction between different ecotypes
316"
(Guinet & Bouvier, 1995; discussion in Hoelzel et al., 1998). In songbirds, local dialects are
317"
often correlated with female acoustic preferences and may lead to reproductive isolation and
318"
ultimately speciation (Price, 2008). Similar processes could be affecting killer whale mating
319"
patterns, but no direct data exist on traits influencing mate preferences or the strength of
320"
behavioral isolation in this species.
321"
Second, cultural specializations could provide an effective barrier for between-culture
322"
dispersal by inducing temporary natural selection against immigrants (Nosil et al., 2005):
323"
potential dispersers would have to adjust behaviorally to the local culture and learn the local
324"
foraging behaviors (Table 2) in order to survive and reproduce with members of that culture
325"
(Danchin & Wagner, 2008, 2010). Until such cultural matching is achieved, dispersing
326"
individuals would experience drastically reduced fitness. Contrary to traditional views on
327"
ecological speciation (Rundle & Nosil, 2005) the reduction in immigrant fitness in killer whales
328"
could be based on learned behaviors rather than genetic adaptations (see also discussion in Pilot
329"
et al., 2010). Whether killer whales are able to acquire non-natal cultural traits throughout their
330"
lifetime or whether there is an age-specific imprinting phase is not known. The failure to
15
331"
successfully reintroduce the long-term captive whale Keiko to the wild suggests that the ability
332"
to correctly assimilate cultural traditions could be age-specific (Simon et al., 2009), but future
333"
studies on this are clearly warranted.
334"
Third, cultural differences can lead to post-zygotic barriers through neutral processes
335"
such as genetic drift rather than selection (Coyne & Orr, 2004). This is particularly likely, given
336"
the small population size of most killer whale ecotypes (e.g., Hoelzel et al., 2007), which
337"
increases both the potential for nonselective changes in allele frequencies as well as the speed of
338"
reproductive isolation arising through genetic drift (Coyne & Orr, 2004).
339"
Finally, cultural differences can lead to post-zygotic barriers via gene-culture coevolution
340"
when cultural innovations change selective pressures acting on a population, eventually leading
341"
to population divergence (Danchin & Wagner, 2008; Laland et al., 2010). For example, if a
342"
single individual within a population invents a new behavior that aids in exploiting a resource
343"
(e.g., intentional stranding: Lopez & Lopez, 1985; Guinet, 1991; Hoelzel, 1991; Guinet &
344"
Bouvier, 1995), this new innovation can spread rapidly within the population within a single
345"
generation as a result of vertical and horizontal transmission via social learning. Hence, cultural
346"
transmission can rapidly decrease within-group variance, while simultaneously increasing
347"
between-group variance (Lehmann & Feldman, 2008). This has the potential to effectively
348"
change the selective pressures acting on cultural groups, so that cultural change could facilitate
349"
or preempt a genetic response just like that described for human enzymes (Simoons, 1978;
350"
Whitehead, 1998; Richerson & Boyd, 2005; Hawks et al., 2007; Perry et al., 2007; Lehmann &
351"
Feldman, 2008; Laland et al., 2010). In killer whales, differences in the dietary metabolism or
352"
respiratory and muscular systems are to be expected between different killer whale ecotypes
353"
(Foote et al., 2011a). For example, a preliminary anatomical analysis suggests that resident and
16
354"
transient killer whales differ in skull morphology, body size, and other features, probably as a
355"
result of selection for robustness in transients to successfully hunt marine mammals (cited in
356"
Reeves et al., 2004). Moreover, the physiological requirements needed to successfully perform
357"
the endurance-exhaustion technique described for killer whales foraging on tuna (Guinet et al.,
358"
2007) are likely different from those needed for foraging on salmon (Ford et al., 1998) or marine
359"
mammals (Miller, Shapiro & Deecke, 2010). Other genetic changes similar to those found in
360"
humans are likely, but they will have to be the focus of future functional genomic research. In
361"
particular, we propose that next generation sequencing approaches could help uncover genes that
362"
are the target of selection in different killer whale ecotypes (see also Foote et al., 2011a).
363"
Candidate genes for this could be identified in model organisms for which the entire genome has
364"
been sequenced (e.g. humans, or mice).
365"
Killer whales of different ecotypes and from different geographic regions successfully
366"
interbreed in captivity (Bowles, Young & Asper, 1988; Duffield et al., 1995), ruling out genetic
367"
incompatibility (i.e., hybrid inviability), at least on the grossest level. However, with the
368"
scientific data available to us, we were unable to discern if other genetic incompatibilities (e.g.,
369"
hybrid sterility) applied to offspring sired by parents of different ecotypes (but see for example
370"
URL 1 for an account of offspring sired by hybrid parents). If killer whale ecotypes have already
371"
evolved certain co-adapted genes, reduced hybrid fitness, for example in the form of lower
372"
growth rates or the inability to process certain food items, could already be possible but would
373"
only become obvious under natural conditions.
374"
Killer whales appear to be a good example of behavioral isolation evolving far ahead of
375"
hybrid inviability or sterility (Coyne & Orr, 2004; see Price, 2008 for a discussion of this
376"
phenomenon in birds). However, in theory matrilineal behavioral preferences may be ephemeral
17
377"
and reversible (see discussion in LeDuc et al., 2008). In fact, reversal of the observed processes
378"
of differentiation due to hybridization or homogenization have been observed between different
379"
ecotypes in other well-established model systems such as sticklebacks and cichlids (Taylor et al.,
380"
2006; Seehausen et al., 2008; Nosil, Harmon & Seehausen, 2009; Behm, Ives & Boughman,
381"
2010), and there is good evidence for gene flow between ecotypes/populations in killer whales as
382"
well (e.g., Hoelzel et al., 2007; Pilot et al., 2010). Nonetheless, recent studies have demonstrated
383"
that speciation is possible despite the presence of gene flow (e.g., Dieckmann & Doebeli, 1999;
384"
Via, 2009; de León et al., 2010). Clearly, different killer whale ecotypes fall into different
385"
positions along the continuum between panmixia and complete reproductive isolation, and none
386"
seem to have yet achieved full reproductive isolation. Because of this, killer whales provide an
387"
interesting case study into the factors that promote or constrain ecotype movement along the
388"
speciation continuum towards ecological speciation (e.g., Hendry, 2009; Nosil et al., 2009). For
389"
example, what will happen if certain populations fall below a critical population size, or if no
390"
individual of breeding age for one sex were available? In other word, whether speciation will in
391"
fact occur depends on whether divergence exceeds the ecotype/population extinction rate. We
392"
could be witnessing the early stages of an adaptive radiation of killer whales whereby a variety
393"
of incipient species are beginning to exploit diverse ecological niches, or conversely, we could
394"
be looking at an old and ongoing process by which periodically new ecotypes form and wink out
395"
(Barrett-Lennard, 2011).
396"
18
397"
CAN CULTURAL TRADITIONS LEAD TO STABLE REPRODUCTIVE
398"
ISOLATION?
399"
Three incidents may help shed some light on this question. The first one is the report of three
400"
transient killer whales captured off British Columbia in 1970. For the first 75 days of captivity,
401"
all three individuals refused to eat the fish provided by their captors, which eventually resulted in
402"
death by starvation of one killer whale. Only after that event did the other two transients begin
403"
eating fish, but they immediately reverted back to foraging on marine mammals after their
404"
release back to the wild (Ford & Ellis, 1999). For two other transients from the same capture an
405"
interesting case of intercultural transmission occurred when they, after 24 days of self-induced
406"
starvation, were put in a pool with a Southern resident and were both feeding on herring within
407"
hours after being passed fish by the resident (G. M. Ellis, unpublished data). The third example
408"
stems from observations made following the ‘Exxon Valdez’ oil spill in 1989. AT1 pod (a small,
409"
genetically distinct population of transients from Prince William Sound, Alaska) lost 9 members
410"
(41%) of their social group including several reproductive females after the spill, and an
411"
additional 4 males after 2000 (Matkin et al., 2008). Since no successful recruitment has taken
412"
place in this social group since 1984, group size is now reduced to 7 individuals with only two
413"
reproductive females and one adult male (Matkin et al., 2008). One possibility enabling group
414"
survival would be for AT1 to join another transient population, for example the Gulf of Alaska
415"
transients; however, there is so far no indication of this happening (Matkin et al., 2008). Thus, it
416"
appears that cultural traditions in killer whales can be a strong reproductive isolation mechanism
417"
that is stable even in the face of individual death or potential population extinction.
418"
19
419"
CONCLUSIONS
420"
Our review reveals multiple trait divergence among killer whales, including population
421"
divergence in behavior, pigmentation patterns, morphology, dietary specializations, and genetics.
422"
While there are still many open questions concerning the degree of this divergence and the
423"
strength of reproductive isolation, the overall pattern that emerges strongly suggests that
424"
ecological speciation could be the driving force behind global killer whale diversity. Dietary
425"
specialization is likely to have resulted in divergent selection between populations thus fulfilling
426"
the first requirement for ecological speciation. Behavioral isolation due to sexual imprinting is
427"
the most likely candidate for a current mechanism for reproductive isolation (the second
428"
requirement; Rundle & Nosil, 2005). Although the currently available data did not allow us to
429"
identify a genetic mechanism linking divergent selection to reproductive isolation, we argue that
430"
this is not strictly necessary. What is needed is a deterministic mechanism that links divergent
431"
selection to reproductive isolation. As an alternative to strict genetic inheritance, another
432"
heritable unit ––culture–– clearly provides this deterministic mechanism in killer whales.
433"
An interesting alternative, however, could be that cultural divergence may predate
434"
ecological divergence in killer whales, meaning that they may not be undergoing ecological
435"
speciation per se, but rather “cultural speciation” as outlined by Gavrilets (2004); a combination
436"
of cultural and ecological mechanisms is also possible.. Future studies are needed to shed more
437"
light on which came first, cultural or ecological divergence.
438"
While killer whales are probably unusual in the extent to which culturally-driven
439"
selection has driven diversification and ultimately speciation (e.g., there is so far no evidence
440"
that culture has led to evolutionary significant levels of reproductive isolation in humans),
441"
culture and behavioral traditions also appear to be important features in the biology of various
20
442"
other cetacean species, such as humpback whales, sperm whales (Physeter macrocephalus), and
443"
bottlenose dolphins (reviewed in Rendell & Whitehead, 2001; Laland & Galef, 2009). Hence, the
444"
combination of cultural and ecological divergence may have been the main driving force behind
445"
some of the observed species diversity in the order Cetacea (Berta, Sumich & Kovacs, 2006).
446"
Most importantly, however, such divergence may not be restricted to cetaceans and to a greater
447"
or lesser degree may be affecting any species (e.g., birds), in which socially transmitted
448"
behaviors have fitness consequences. While most of that divergence took place in the past, killer
449"
whales provide an excellent opportunity to study patterns of divergence and diversification in
450"
action. However, these patterns of current ecotype diversity in killer whales also have important
451"
implications for conservation, because they raise questions about the evolutionary significant
452"
units that warrant protection (e.g., Moritz, 1994; Hoelzel, 1998; Ryan, 2006; Whitehead, 2010).
453"
Because many of their behavioral traits, such as vocal dialects, can be easily quantified,
454"
killer whales provide an exceptional opportunity to investigate how cultures can affect the
455"
evolutionary trajectories of populations, an aspect often ignored in the conceptual literature on
456"
speciation processes (but see Coyne & Orr, 2004; Price, 2008). Recent studies have
457"
demonstrated that culture plays an important role in shaping human evolution via culture-gene
458"
coevolution (Simoons, 1978; Richerson & Boyd, 2005; Hawks et al., 2007; Perry et al., 2007;
459"
Laland et al., 2010). However, the evidence we provide for killer whales suggests that culture
460"
and behavioral traditions could have far greater evolutionary consequences than previously
461"
assumed (see also Boran & Heimlich, 1999; Baird, 2000). Following the example of Danchin &
462"
Wagner (2010), we therefore propose that the cultural component of behavior should be included
463"
along with phenotypic plasticity and epigenetics in a revised form of the modern synthesis of the
464"
study of evolution (see also Pigliucci, 2007; Pennisi, 2008). Furthermore, we propose that the
21
465"
third component of ecological speciation be extended to include both genes and culture as
466"
mechanisms linking divergent selection and reproductive isolation.
467"
Finally, recent years have seen the emergence of more specific theoretical models on how
468"
ecologically-based divergent natural selection can result in population divergence and ultimately
469"
speciation (e.g., Gavrilets et al., 2007; Sadedin et al., 2009). Future research on killer whales
470"
could—to the extent that this is possible with field studies on such a large marine organism—try
471"
to specifically test some of these models and their predictions.
472"
473"
ACKNOWLEDGEMENTS
474"
We thank R. R. Dunn, R. A. Martin, R. B. Langerhans and the Langerhans lab, as well as M.
475"
Plath, I. Schlupp, and four anonymous reviewers for valuable comments and discussions that
476"
helped improve previous versions of this manuscript. We are greatly indebted to Uko Gorter for
477"
allowing us to use his illustration of different killer whale ecotypes. S. E. Diamond helped create
478"
Figure 2.
479"
480"
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34
793$
Table 1. Divergent traits between identified killer whale ecotypes.
Region!
NE Pacific
Antarctica
Ecotype comparison
Divergent traits
AB
AS
D/F
G
M
MP
P
SS
Estimated time
since divergence
(mtDNA)
residents vs. transients
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
~700.000 yrs
Baird &
Whitehead,
2000; Deecke et
al., 2005;
Barrett-Lennard,
2006; Morin et
al., 2010; Pilot et
al., 2010; Riesch
& Deecke, 2011;
Ford & Ellis, in
press
residents vs. offshores
n/a
Yes
Yes
Yes
Yes
Yes
Yes
Yes
>150.000 yrs
Barrett-Lennard
& Heise, 2006;
Dahlheim et al.,
2008; Morin et
al., 2010; Pilot et
al., 2010; Ford et
al., 2011; Ford
& Ellis, in press
transients vs. offshores
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
~700.000 yrs
Barrett-Lennard
& Heise, 2006;
Dahlheim et al.,
2008; Morin et
al., 2010; Pilot et
al., 2010; Ford et
al., 2011; Ford
& Ellis, in press
Type A vs. Type B (Pack
Ice)
n/a
n/a
?
Yes
n/a
n/a
Yes
?
>150.000 yrs
Pitman & Ensor,
2003; Morin et
al., 2010;
Pitman, 2011
35
References
Type A vs. Type B
(Gerlache)
n/a
n/a
?
Yes
n/a
n/a
Yes
?
>150.000 yrs.
Pitman & Ensor,
2003; Morin et
al., 2010;
Pitman, 2011
Type A vs. Type C
n/a
n/a
Yes
Yes
Yes
?
Yes
Yes
>150.000 yrs
Pitman &
Ensor, 2003;
Pitman et al.,
2007; Morin et
al., 2010
Type A vs. Type D
n/a
n/a
?
n/a
Yes
?
Yes
n/a
n/a
Pitman et al.,
2011
Type B (Pack Ice) vs. Type
B (Gerlache)
n/a
n/a
Yes
n/a
n/a
n/a
n/a
Yes
n/a
Pitman, 2011
Type B (Pack Ice) vs. Type
C
n/a
n/a
Yes
Yes
Yes
?
Yes
Yes
~150.000 yrs
Pitman & Ensor,
2003; Pitman et
al., 2007;
Andrews et al.,
2008; Morin et
al., 2010;
Pitman, 2011
Type B (Pack Ice) vs. Type
D
n/a
n/a
?
n/a
Yes
?
Yes
n/a
n/a
Pitman et al.,
2011; Pitman,
2011
Type B (Gerlache) vs. Type
C
n/a
n/a
Yes
Yes
Yes
?
Yes
Yes
~150.000 yrs
Pitman & Ensor,
2003; Pitman et
al., 2007;
Andrews et al.,
2008; Morin et
al., 2010;
Pitman, 2011
36
NE Atlantic
Type B (Gerlache) vs. Type
D
n/a
n/a
?
n/a
Yes
?
Yes
n/a
n/a
Pitman et al.,
2011; Pitman,
2011
Type C vs. Type D
n/a
n/a
n/a
n/a
Yes
?
Yes
n/a
n/a
Pitman et al.,
2011; Pitman,
2011
generalists vs. mammaleaters
n/a
n/a
Yes
n/a
Yes
Yes
Yes
n/a
n/a
Similä et al.,
1996; Wolkers et
al., 2007; Foote
et al., 2009,
2011c; Morin et
al., 2010
794$
AB: acoustic behavior; AS: acoustic signals; D/F: diet and foraging behavior; G: genetics, M: morphology; MP: movement/diving
795$
patterns; P: pigmentation; SS: social structure; RI: reproductive isolation; ?: inconclusive, but preliminary studies suggest divergence;
796$
n/a: data not available
37
Table 2. Cultural traditions of killer whale populations from around the globe.
Phenomenon!
Behavioral
context
Geographic Region
References
Call dialects
all behaviors
NE Pacific
Ford, 1989, 1991;
Yurk et al., 2002;
Deecke et al., 2005;
Ford & Ellis, in press
Greeting ceremonies
social behavior
NE Pacific
Osborne, 1986
Whistle dialects
social behavior
NE Pacific
Riesch et al., 2006;
Riesch & Deecke,
2011
Beach rubbing
social behavior
NE Pacific
Ford, 1989; Ford,
Ellis & Balcomb,
2000
Benthic foraging
foraging behavior
SW Pacific
Visser, 1999
Carousel feeding
foraging behavior
NE Atlantic
Similä & Ugarte,
1993; Domenica et
al., 2000; Nottestad
& Similä, 2001;
Simon et al., 2005
NW Pacific
Tarasyan et al., 2005
Endurance-exhaustion
hunting
foraging behavior
Atlantic
Guinet et al., 2007
Exploitation of longline
fishing
foraging behavior
widespread
Secchi & Vaske,
1998; Nolan &
Liddle, 2000; Visser,
2000
Intentional stranding
foraging behavior
SW Atlantic
Lopez & Lopez,
1985; Hoelzel, 1991
S Indian Ocean
Guinet, 1991; Guinet
& Bouvier, 1995
Southern Ocean
Smith et al., 1981;
Visser et al., 2008;
Pitman, 2011 ;
Pitman & Durban, in
press
Wave-washing
foraging behavior
!
38
FIGURE LEGENDS
Figure 1. World map with approximate distributions of known sympatric killer whale ecotypes
in red and potential sympatric ecotypes in eggshell.
Figure 2. Known killer whales ecotypes of the northern hemisphere. Note the size, pigmentation,
and dorsal fin shape differences between the sexes (males left, females right), but also between
ecotypes. Artwork by Uko Gorter.
Figure 3. Known killer whales ecotypes of the southern hemisphere. Note the size,
pigmentation, and dorsal fin shape differences between the sexes (males left, females right), but
also between ecotypes. Artwork by Uko Gorter.
39